asymmetric and asynchronous energy conservation protocol for vehicular networks

ABSTRACT

An asymmetric and asynchronous energy conservation protocol for vehicular networks is provided. In one aspect, an a-quorum may be defined for one or more members for the one or more members to establish asymmetric links to contact the cluster head. An s-quorum for the cluster head to establish symmetric link between the cluster heads and relays may be defined.

FIELD OF THE INVENTION

The present application relates generally to Dedicated Short RangeCommunications (DSRC), Intelligent Transportation Systems (ITS),vehicular network, wireless communication, energy conservation, quorumsystem, and more particularly to an asymmetric and asynchronous energyconservation protocol for vehicular networks.

BACKGROUND OF THE INVENTION

Intelligent transportation systems (ITS) encompass a broad range ofwireless and wire line communications-based information and electronicstechnologies. Integrating ITS into the transportation system'sinfrastructure and in vehicles may improve safety on the roads, e.g.,passenger and pedestrian safety, improve transportation productivitythrough the use of vehicle-to-vehicle and vehicle-to-roadside wirelesscommunication technologies, and relieve congestion. For instance,investigations report that 17-37% of car accidents and 60% of roadwaycollisions could be prevented with development of the IntelligentTransportation Systems (ITS) technologies.

Due to the high mobility of vehicles, however, communications invehicular networks need to satisfy strict delay requirements, which areusually less than a few hundreds of milliseconds. The United StatesFederal Communications Commission (US FCC) has approved 5.9 GHz spectrumfor Dedicated Short Range Communications (DSRC) aiming to providereal-time, high speed links for vehicular networks. DSRC aims to supportshort duration wireless communications in rapid changing environments.Currently, the ASTM E2213-03 standard is being migrated to the IEEE802.11p standard, and a new operation mode, named Wireless Access inVehicular Environments (WAVE) mode, is added to IEEE 802.11.

ITS deployments require each vehicle to be equipped with an On BoardUnit (OBU) performing wireless communications with the other vehicles orRoad Side Units (RSUs). Briefly, equipment in the DSRC service comprisesOBUs and RSUs. An OBU may be a transceiver that is installed in or on avehicle, or a portable unit. An RSUs may be a transceiver that ismounted along a road or pedestrian passageway. An RSU may also bemounted on a vehicle or hand carried, but it may only operate when thevehicle or hand-carried unit is stationary. Am RSU broadcasts data toOBUs or exchanges data with OBUs in its communications zone.

Certainly, the installation of OBUs on modern vehicles burdens theever-increasing fuel consumption over the automotive electronic devices.Besides, the market has seen an increasing adoption of portablenavigation devices (e.g., PDA) which may include OBU functions in thefuture. As significant gains in energy conservation are not likely toadvent through refinement of the mature drivetrain (e.g., internalcombustion engine) or battery technologies, there is a need for anenergy-conservative communication protocol that improves thefuel/battery economy.

One direct way for DSRC to achieve fuel/energy conservation is toinherit the Power Saving (PS) mode from IEEE 802.11 standard. In IEEE802.11 PS mode, the time axis on a station (in this case, OBU) isdivided evenly into beacon intervals. Beacon interval in IEEE 802.11refers to the amount of time between beacon transmissions. Before astation enters power save mode, the station uses the beacon interval toknow when to wake up to receive the beacon. IEEE 802.11 PS mode allowsan idle station to sleep a portion of each beacon interval. In IEEE802.11 PS mode, an auxiliary timer synchronization mechanism is requiredto ensure the overlap of awake periods. Two power saving vehicles cancommunicate with each other only when their timers are synchronized.Since the topology of a vehicular network is frequently partitioned,making the timer synchronization costly or even infeasible, the IEEE802.11 PS mode may not be practicable in vehicular networks.

Based on the IEEE 802.11 PS mode, a number of studies have exploredAsynchronous Quorum-based Power Saving (AQPS) protocols. In an AQPSprotocol, a station may stay awake

Given a-quorum or s-quorum, a station shall stay awake at those beaconintervals whose numbers are specified in the quorum. Since this quorumcan overlap with another quorum or quorums (depending on its type “a” or“s”), the station can have a chance to discover and communicate withother stations at the overlapped beacon interval, therefore stayconnected to the whole network. The overlap between quorums ensuresnetwork connectivity.

In clustered vehicular networks, the clusterheads, relays, and RSUs canuse s-quorums to establish symmetric links (FIG. 1, 102, 104) betweenthemselves; while members can use a-quorums to establish asymmetriclinks (FIG. 1, 106) to contact their clusterheads. Stations adoptings-quorums are able to discover each other as in conventional quorumsystems. The asymmetric quorum system guarantees the intersectionbetween every pair of s-quorums. Stations adopting a-quorums, however,can only discover stations with s-quorums. The asymmetric quorum systemdoes not guarantee intersection between a-quorums. Once two stationsboth stay awake in a beacon interval, they can discover each other,i.e., to remember the schedule of each other, and predict when the nextawake beacon interval should arrive at the receiving station if it wantto send data. In this way, a “link” is considered established betweenthese two nodes. In this disclosure, “a” is used to indicate that somelink can be establed only between members and clusterheads, but notbetween members themselves.

Notice that in clustered vehicular networks, the majority of thevehicles are members. Since these members adopt a-quorums to contacttheir clusterheads, the small-sized a-quorums, as given in Eq. (1), canyield significant gain in energy saving.

Conventional AQPS protocols adopt symmetric quorum systems to definecycle patterns for all nodes in a network. The system and method of thepresent disclosure, on the other hand, differentiates the cycle patternsof members and clusterheads (or relays, RSUs) by using an asymmetricquorum system. The asymmetric quorum system defines a-quorums ands-quorums for the members and clusterheads (or relays, RSUs)respectively, and therefore the duty cycles of members and clusterheads(or relays, RSUs) are differentiable without losing the network or sleepduring each beacon interval. Given an integer n, a quorum system definesa cycle pattern, which specifies the awake/sleep schedule during ncontinuous beacon intervals, for each station. Herein, n is referred toas the cycle length since cycle patterns repeat every n continuousbeacon intervals. AQPS protocols ensure the asynchronous overlap ofawake periods between stations; that is, during each cycle, an awakebeacon interval of a station is guaranteed to overlap that of anotherstation, and data communications can be successfully performed at theseoverlapped intervals, even if their boundaries are not aligned.

While AQPS protocols require no timer synchronization mechanism and mayexhibit better feasibility for wide-scale, high-mobility vehicularnetworks, in most AQPS protocols the degree of power saving is limitedby a theoretical bound. Given a cycle length n, a station is required toremain fully awake at least √{square root over (n)} beacon intervals percycle to preserve the asynchronous overlap. The duty cycle of a station(i.e., portion of time a station must remain awake) can be no less thanO(√{square root over (n)}/n)=O(1√{square root over (n)}). Note theneighbor discovery time increases proportionally to n due to the factthat the overlap may occur merely once every n beacon intervals. Inpresence of high mobility, the value of n should be set small to allowvalid neighbor maintenance on each station. Under such a condition, thelower-bound of duty cycle can seriously restrict the effectiveness of anAQPS protocol.

It remains a challenge to efficiently assemble a symmetric quorum systemwhile keeping the quorum size small. Existing symmetric quorum systemsare constructed by using exhaustive searches or assuming n=k²+k+1, wherek is a prime power. To efficiently construct an asymmetric quorum system(i.e., a-quorums and s-quorums) is even more challenging.

BRIEF SUMMARY OF THE INVENTION

A method and system for an asymmetric and asynchronous energyconservation protocol for vehicular networks are provided. The method inone aspect may include defining an a-quorum for one or more members of acluster, the cluster including a plurality of moving vehicles in avehicular network and in which the plurality of moving vehicles aredesignated as a member, a cluster head) or a relay, the a-quorumspecifying a beacon interval for one or more members of the cluster toestablish asymmetric links to contact a cluster head of the cluster. Themethod may also include defining an s-quorum for the cluster head toestablish symmetric link between the cluster head and one or more ofother cluster heads in one or more other clusters.

A system for an asymmetric and asynchronous energy conservation protocolfor vehicular networks, in one aspect, may include a plurality ofstations associated with respective one or more vehicles in a vehicularnetwork. The plurality of stations are organized into one or moreclusters, and assigned as one of a member, cluster head, or relay in arespective cluster. An a-quorum specifies a beacon interval for one ormore members of a cluster to establish asymmetric links to contact acluster head of the cluster, and an s-quorum define a set of beaconintervals for the cluster head to establish symmetric link between thecluster head and one or more of other cluster heads in one or more otherclusters.

A program storage device readable by a machine, tangibly embodying aprogram of instructions executable by the machine to perform the methodsdescribed herein may be also provided.

Further features as well as the structure and operation of variousembodiments are described in detail below with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates symmetric/asymmetric links in clustered vehicularnetworks.

FIGS. 2A and 2B illustrate the operation of IEEE 802.11 Power Saving(PS) mode.

FIG. 3 illustrates two awake/sleep schedules given by a grid quorumscheme.

FIG. 4A illustrates that Asynchronous Majority Quorum (AMQ) of thepresent disclosure in one embodiment ensures that all projections ofA(α) and S(α, β) onto a modulo-(α−1) plane forms an (α−1)-bicoterie.

FIG. 4B illustrates that AMQ ensure that all projections of S(α, β) ontoa modulo-(β−1) plane forms an (β−1)-bicoterie.

FIGS. 5A and 5B illustrate how α and β may affect the duty cycle on aclusterhead or member.

FIG. 6 illustrates that DSRC-AA guarantees the worst case neighbordiscovery time of symmetric and asymmetric links.

FIGS. 7A and 7B illustrates comparison of the duty cycles over specificrequirements on neighbor discovery time.

DETAILED DESCRIPTION

This disclosure describes a new energy conservation protocol for DSRC,named DSRC Asymmetric and Asynchronous wakeup (DSRC-AA), which reducesduty cycles and improves energy efficiency as compared with traditionalAQPS protocols while ensuring real-time link access. In one aspect,DSRC-AA is based on IEEE 802.11 and provides power saving to thecommunication module in vehicles (e.g., a built-in OBU or a portabledevice), while ensuring a bounded delay overhead.

The following notations are used in the disclosure:

-   Symbol Description-   BI duration of a beacon interval-   AW duration of an ATIM window-   n the cycle length-   δ time shift remainder (in each BI)-   U the universal set-   Q a quorum-   α tolerable neighbor discovery time (in number of BI s) for    Vehicle-To-Vehicle (V2V) asymmetric links-   β tolerable neighbor discovery time (in number of BI s) for    Vehicle-To-Roadside (V2R) or V2V symmetric links-   A(α) a generating set for a-quorums-   S(α, β) a generating set for s-quorums

There are two network scenarios described in DSRC: the distributed andcentralized networks. The distributed network is formed by the On boardUnits (OBUs) that gives ad hoc connectivity between vehicles; while thecentralized network is formed by the Road Side Units (RSUs) offeringvehicles the wired backbone access.

FIG. 1 illustrates symmetric and asymmetric links in clustered vehicularnetworks. In a driving scenario, vehicles moving in the same direction,for example, following the lane direction, have low relative speed. Dueto their low relative mobility, vehicles moving toward the samedirection may have connection periods longer than 10 seconds. Thetopology among these vehicles is stable. Those vehibles may be organizedas a cluster.

FIG. 1 illustrates two clusters 102, 104 of vehicles moving in lowrelative speed to one another. In each cluster, a temporary clusterheadmay be elected (e.g., 106, 108), which gathers information from nearbyvehicles in the cluster and coordinates operations (e.g., collisionavoidance, emergency warning, or highway platooning). In one aspect,each vehicle may have a functional role of clusterhead, relay, ormember. The clusterhead normally serves as the local coordinator in acluster that ensures consisten, reliable, and sequenced reactions totraffic changes. The relay forwards data within a cluster or betweenclusters. The member is an ordinary node that communicates only with theother vehicles in the same cluster. Compared to the flat structure,clustering allows efficient mobility management, locatlization of nodedynamics, and better network scalability. Such a clustering or groupingconcept allows the sequenced and consistent reactions to traffic changeswhile avoiding message flooding. DSRC provides vehicle-to-vehicle and/orvehicle-to-roadside links lasting 200 to 300 meters line of sightypically.

In a cluster (e.g., 102, 104), each member (i.e., regular vehicle) mayrely on the clusterhead to forward its awake/sleep schedule or data.There is no need to maintain the overlap of awake intervals betweenevery pair of vehicles. In other words, it is sufficient to ensure thatthe awake periods of each member in a cluster will overlap with those ofthe clusterhead (e.g., 102, 104).

DSRC-AA in one embodiment employs a new type of quorum system, namedAsymmetric Majority Quorum (AMQ) system, tailored for vehicularnetworks. The AMQ system defines two types of cycle patterns for membersand the clusterhead respectively. These cycle patterns guarantee theoverlap of awake beacon intervals between each member and theclusterhead in a cluster, and between all clusterheads in a network. Theconstruction of cycle patterns for the member and clusterhead is basedon the specific delay requirements α and β respectively, where α denotesmaximum allowable latency in vehicle-to-vehicle communications and βdenotes the allowable latency in vehicle-to-roadside communications.Since vehicles in the same cluster remain relatively stable in topology,it is assumed that α≧β, based on which the duty cycles of a member andits clusterhad may be differentiated. According to the system and methodof the present disclosure in one embodiment, when a heavier duty cycleis taxed on the clusterhead to meet the strict β, each member in acluster can have lighter duty cycle below the O(1/√{square root over(n)}) bound, without losing the performance guarantee to α. Sincemembers are the majority of vehicles, DSRC-AA enables substantialreduction in average energy consumption.

In one embodiment, DSPC-AA defines a-quorums and s-quorums for themembers and clusterheads (or relays, RSUs) respecively. Quorumsgenerally refer to sets in which a pair of quorums has some elements incommon. In one embodiment, asymmetric links are established betweenmembers and their clusterhead. In this disclosure, members are alsoreferred to as member nodes. Symmetric links are established betweenclusterheads and relays. Consider two positive integers α and β whichmay denote the maximally tolerable neighbor discover time (in terms ofthe number of beacon intervals) of the asymmetric and symmetric linksrespectively. The two parameters may set, for instance by a systemadministrator and received as input parameters.

We assume α≧β since vehicles in the same cluster are relatively stableas compared with their absolute moving speed. Let n=└(α−1)/2┘ be thecycle length of members and {0,1, . . . ,n−1} be the set of numbers ofbeacon intervals in a cycle, the a-quorums, A(α), is defined as follows:

A(α)={0}.   (1)

This implies that each member should remain awake during the firstbeacon interval of a cycle. That is, during beacon intervals 0 to n-1, astation needs to stay awake only at the 0-th beacon interval.

Let m=└(α−1)/2┘+└(β−1)/2┘−1 be the cycle length of the clusterheads (orrelays, RSUs) and {0,1, . . . ,m−1} be the set of numbers of beaconintervals in a cycle, the s-quorums, S(α,β), is defined as follows:

S(α,β)={0,1, . . . ,└(α−1)/2┘−1}.   (2)

That is, during beacon intervals 0 to m−1, a station needs to stay awakeonly at the 0-th, 1, . . . ,floor((alpha-1)/2)−1 beacon intervals. Cyclelength in the present disclosure refers to the number of beaconintervals for which the wake-up and sleep schedule repeats on a station.

In one embodiment, DSRC-AA inherits the beacon structure as given inIEEE 802.11 PS mode and in J. R. Jiang, Y. C. Tseng, C. S. Hsu, and T.H. Lai. Quorum-based asynchronous power-saving protocols for ieee 802.11ad hoc networks. Mobile Networks and Applications, 10(1-2);169-181,2005. During the beacon intervals whose numbers are specified in thequorum, a station is required to remain fully awake. During the rest ofbeacon intervals, the station may sleep after the Ad hoc TrafficIndication Map (ATIM) window. Briefly, ATIM are used in IEEE 802.11 toannounce the existence of buffered frames. These messages are sentbetween wireless stations to prevent them entering power saving mode andto indicate there is data to follow. connectivity. Eq. (1) above showsthat the cardinality of an a-quorum can be arbitrarily small(specifically, O(1)-sized). Therefore, the degree of power saving isexpected to be substantially improved.

The design of DSRC-AA takes into account several practical issues.Although there may not be an intersection between those stations thatadopt a-quorums, that does not imply that members are not able todirectly communicate with each other. For instance, although stationscan adopt the same a-quorum, their clock may shift. For example, beaconinterval 0 on station X may be beacon interval 3 on station Y whenstation Y's clock is two-interval-faster than X. Under such a situation,the two a-quorums do not intersect either from X's or Y's point of view.Rather, a clusterhead knows the schedule of each member in a clusterthrough asymmetric links, and therefore, for example, can piggyback themember's schedule in its own beacon frames. By listening to the beaconframes of the clusterhead carrying all members' schedules, members areable to obtain one another's schedule and predict the coming ATIM windowat the receiving party. Further, under the situation that a cluster isforming or reforming, or the clusterhead is lost, members cantemporarily adopt s-quorums until a clusterhead is elected. For example,if a member does not receive the beacon frame of its clusterhead for along time (longer than a threshold), it then may adopt the s-quorumdirectly; that is, stay awake for those beacon intervals whose numbersare specified in the s-quorum.

In one embodiment of the present disclosure, to resolve the possiblefairness issue on fuel/energy consumption if the asymmetric quorumsystem poses heavier duty cycles on vehicles using s-quorums, the systemand method of the present disclosure may allow the clusterhead to eitherspecify a successor or trigger a re-election after its serving age.

With the beacon structures using the symmetric and asymmetric quorumsystem of the present disclosure, two stations H₀ and H₁ adopting S(α,β)and S(α′,β), α, α′≧β, as quorums respectively are able to discover eachother within β beacon intervals despite their clock shifts. Forinstance, alpha (α) and alpha prime (α′) may represent two different“tolerance” parameters. Even if two stations adopts quorums constructedbase on different alpha values, they are still guaranteed to both remainawake at some beacon interval, despite of their clock shift. Also, twostations H₀ and H₁ adopting A(α) and S(α′,β), β≦α≦α′, as quorumsrespectively are able to discover each other within a beacon intervalsdespite their clock shifts. Thus the connectivity of a network may beensured. Further, since in DSRC-AA a station remains awake during everyATIM window, the data buffering delay is bounded by a beacon interval.DSRC-AA renders great feasibility for vehicular networks since itimproves the overall energy efficiency while ensuring a bounded neighbortime and data buffering delay.

The following briefly describes IEEE 802.11 PS mode.

The operation of IEEE 802.11 Power Saving (PS) mode is shown in FIG. 2A.On each PS station, the time axis is divided evenly into beaconintervals. In every beacon interval, the station is required to remainawake during the entire Announcement Traffic Indication Message (ATIM)window. A beacon frame is broadcasted at the Target Beacon TransmissionTime (TBTT) to announce the existence of network. If a station, e.g.,H1, intends to transmit data to the destination H0 (FIG. 2A, shown at202), H1 first unicasts an ATIM frame to H0 during the ATIM window (FIG.2A, shown at 204). Remaining awake, H0 receives the ATIM frame and sendsback an acknowledgement. Both H0 and H1, after this ATIM notificationprocedure, keep awake for the entire beacon interval. After the end ofATIM window, the DCF (Distributed Coordination Function) mechanism(e.g., RTS, CTS, and random back-off) are initiated to transmit the datawhile avoiding collisions (FIG. 2A shown at 206). If there are no ATIMnotifications, PS stations may enter the doze mode (that is, to sleep)after each ATIM window. Since AW (i.e., duration of an ATIM window) isone fourth of BI (i.e., duration of a beacon interval), this protocol isable to yield up to 75% energy saving on idle stations.

However, the IEEE 802.11 PS mode is functional only when the timers onstations are synchronized FIG. 2B shows an example where the ATEM framesof H0 and H1 are lost due to their unsynchronized clocks. Apparently,communications between stations can take place only after timersynchronization. In vehicular networks, synchronizing the clocks betweenOBUs through infrastructures may get costly since the connection periodof vehicle-to-roadside links is usually short. Synchronizing throughvehicle-to-vehicle links may not be feasible because the distributednetwork is subject to frequent partition. Stations operating in the WAVEmode are not required to synchronize their timers. This raises a needfor an asynchronous energy conservation protocol.

The following describes the AQPS Protocols.

The Asynchronous Quorum-based Power Saving (AQPS) protocols ensure theoverlap of awake periods between stations by using the quorum systems(e.g., grid, torus, and cyclic difference set). The grid/torus-basedAQPS protocols are summarized as follows. FIG. 3 illustrates twoawake/sleep schedules given by a grid quorum scheme with cycle lengthn=9. A grid quorum scheme numbers every n continuous beacon intervalsfrom 0 to n−1 and organizes them as an √n×√n array in a row-majormanner. It defines a quorum as a set containing all numbers along acolumn and a number from each of the remaining columns (e.g., {0. 1, 2,3, 6} or {2, 3, 4, 5, 8} as shaded in FIG. 3). Each station, by usingthis scheme, is able to obtain its own quorum with a uniform quorum size(i.e., the cardinality of quorum set) 2√n−1. During the beacon intervalswhose numbers are specified in the quorum, a station is required toremain fully awake. During the rest of beacon intervals, the station maysleep after the ATIM window as in the IEEE PS mode if there is no datatransmission. Since the schedule repeats every n beacon intervals, wecall it cycle pattern (or cycle for short). A station with this schememay have duty cycle

$\frac{\left. {{\left( {{2\sqrt{n}} - 1} \right)\overset{\_}{BI}} + {\left( {n - 2} \right)\sqrt{n}} + 1} \right)\overset{\_}{AW}}{n\overset{\_}{BI}}.$

In AQPS protocols, a beacon frame carries information about the schedule(e.g., the quorum set and number of current beacon interval, etc.).Unlike IEEE 802.11 PS mode where a station should cancel its own beacontransmission upon hearing the first beacon frame, each station persistsits beacon transmission (even when others' beacons are heard) in orderto claim its own schedule.

Each quorum in the gird/torus quorum scheme intersects with any otherquorum in at least two elements. This implies that the ATIM windowsbetween stations can overlap at

least twice per cycle, if there is no clock shift. The grid/torus quorumscheme is able to further guarantee that the beacon frame of a stationwill be heard by its neighbors within a cycle even when their clocksshift. FIG. 3 shows an example where H1'S clock leads H0's by 2 -δbeacon intervals. The mutual beacon reception is marked by FIG. 3 at 302and 304. Once the beacon frame is heard, the sending station is able todiscover the receiving station (that is, to recognize the schedule ofreceiving station) and predict its next coming of ATIM window.

Suppose H0 has data for H1 (FIG. 3 at 306), it waits until H1 wakes upand unicasts an ATIM frame to H1 to start the data transmissionprocedure described previously in the IEEE 802.11 PS mode (FIG. 3 at308). If data transmission cannot complete within a single beaconinterval (e.g., due to collisions or large data volume), H0 can set thehas-more-data field in frame header true telling H1 to remain awakeduring the entire successive beacon interval (FIG. 3 at 110). Datatransmission may continue then (FIG. 3 at 112).

The power saving advantage provided by the IEEE PS mode and AQPSprotocols comes at the price of delay. Such delay includes the databuffering time, i.e., duration between a packet arrival (on a sendingstation) and its start of DCF. The data buffering time in IEEE PS modeand AQPS protocols is bounded within a BI, which equals 100 ms bydefault. Note in AQPS protocols, two adjacent stations may not be ableto discover each other until a cycle goes by. The neighbor discoverytime, i.e., the time for a station to discover its new neighbor, istherefore O(n KI) in the worst case. This implies that the value of nshould be small in vehicular networks to ensure the valid neighbormaintenance. For example, consider a vehicle moving in speed 20 m/stoward a tolling RSU with radio range 15 meters. If half of theconnection period is used for tolling, the RSU must discover thevehicle's OBU within

$\left\lbrack {\frac{1}{2} \cdot \frac{2 \cdot 15}{20}} \right\rbrack = 0.7$

seconds. The value of n can be no larger than

$\frac{0.7 \cdot 1000}{\overset{\_}{BI}} = {7{\left( {\overset{\_}{BI} = {100\mspace{14mu} {ms}}} \right).}}$

In this case, only the √4×√4 array is applicable to the grid quorumscheme, giving the duty cycle

$\frac{{\left( {{2\sqrt{4}} - 1} \right) \cdot 100} + {\left( {4 - {2\sqrt{4}} + 1} \right) \cdot 24}}{4 \cdot 100} = {0.81\left( {\overset{\_}{AW} = {25\mspace{14mu} {ms}}} \right)}$

and 19% power saving on an idle station. Most existing AQPS protocolshave limited effect when n is small since their resulted duty cycle canbe no less than O(√n/n)=O(1/√n).

Details of the quorums and protocols of the present disclosure describedabove are provided as follows. As described above, the DSRC-AA of thepresent disclosure employs a new quorum system, named asymmetric quorumsystem, to ensure the overlap of awake periods. The asymmetric quorumsystem defines two types of quorums: the s-quorums (symmetric quorums)and a-quorums (asymmetric quorums). In clustered vehicular networks, theclusterheads, relays, and RSUs can use s-quorums to establish symmetriclinks between themselves; while members can use a-quorums to establishasymmetric links to contact their clusterheads. Stations adoptings-quorums are able to discover each other as in conventional quorumsystems. The asymmetric quorum system guarantees the intersectionbetween every pair of s-quorums. In one embodiment, stations adoptinga-quorums, however, can only discover stations with s-quorums. Theasymmetric quorum system does not guarantee intersection betweena-quorums. In the methodology of the present disclosure, the cardinalityof an a-quorum can be arbitrarily small (specifically, (1)-sized).Therefore, the degree of power saving is expected to be substantiallyimproved.

The following provides formal definitions of an asymmetric quorumsystem.

Asymmetric Quorum System

Consider the sets in which each element denotes a number of beaconinterval.

Definition 3.1 (n □-coterie): Given an integer □ n and a universal set□U={0 □1 □n □−1}. Let X be a set of nonempty subsets of U. We call X ann-coterie if and only if for all

Q, Q′εX, □Q∩Q′≠Ø.

Conventionally, the coterie X □ is termed quorum system, and theelements of X (i.e., Q) are called quorums. Stations adopting thequorums of an n -coterie are able to discover each other once every nbeacon intervals, implying the worst-case neighbor discovery time O(nBI). In presence of clock shift between stations, an n-coterie can stillguarantee the O(n BI) worst-case neighbor discovery time if it is cycle.The above definition serves as the basis for most quorum schemes used inexisting AQPS protocols. For example, the set {{0 □1 □2 □3 □6}, {1 □2 □3□4 □7}} shown in FIG. 3 is a 9-coterie. These quorum schemes are to assymmetric in the sense that they ensure the intersection between everypair of quorums.

In the present disclosure, we generalize the definition of a coterie todefine an asymmetric quorum system.

Definition 3.2 (n-bicoterie): Given an integer n and a universal setU={0, 1, . . . , n−1}. Let X and Y be two sets of nonempty subsets of Urespectively. The pair (X,Y) is called an n-bicoterie if and only if forall QεX and Q′εY, Q∩Q′≠Ø.

Note (X, X) is a bicoterie if and only if □ is a coterie.

Definition 3.3 (n-asymmetric quorum system): Given an integer n and auniversal set U={0, 1 . . . , n−1}. Let X and Y be two sets of nonemptysubsets of U. The ordered pair (X, Y) is called an n-asymmetric quorumsystem if and only if (a) (X,Y) is an n-bicoterie; (b) Y is ann-coterie.

The elements of X and Y are called the a-quorums and s-quorumsrespectively. The asymmetric quorum system defined above is similar tothe read-write quorum systems used in replication management. Differentfrom traditional (i.e., coterie-based) quorum systems, an asymmetricquorum system does not guarantee intersection between a-quorums. In anasymmetric quorum system, given the same cycle length n, the size ofa-quorums can be substantially smaller than that of traditional quorums.The a-quorum size is not lower bounded by O(√n) as in traditional quorumsystems. When a-quorums are applied to the members of a cluster, eachmember may have a duty cycle below O(√n/n)=O(1/√n), yielding significantgain in energy saving. Note that in such a case, the worst case databuffering time and neighbor discovery time remain unchanged ( BI and nBI respectively). The reduction in members' duty cycle does not induceany cost over the worst case delay.

Recall that vehicles in the same cluster are relatively stable intopology as compared with their absolute moving speeds. The maximumallowable neighbor discovery time need not be identical betweenvehicle-to-vehicle and vehicle-to-roadside communications. To reflectthis fact, we may further generalize the definition of an asymmetricquorum system.

Defintion 3.4 (c-truncation): Given an integer c and a set Q. We callT_(c)(Q) a c-truncation of Q if and only if T_(c)(Q)={q: 0≦q≦c−1, qεQ}.

Let X be a set of nonempty subsets of U, U={0, 1, . . . , n−1}. Forconvenience, we denote T_(m)(X)={T_(m)(Q): ∀QεX}.

Definition 3.5 ((n_(a), n_(s))-asymmetric quorum system): Given twointegers n_(a) and n_(s). Let X and Y be sets of nonempty subsets ofU_(x) and U_(y) respectively, where U_(x)={0, 1, . . . , n_(x)} andU_(y)={0, 1, . . . , n_(y)}, n_(x), n_(y)εZ. The ordered pair (X, Y) iscalled an (n_(a), n_(s))-asynmetric quorum system if and only if (a) Tn_(a) (X), T n_(a) (Y)) is an n_(a)-bicoterie; (b) T n_(s) (Y) is ann_(s)-coterie.

An (n_(a), n_(s))-asymmetric quorum system guarantees the worst caseneighbor discovery time n_(a) BI and n_(s) BI for the asymmetric andsymmetric links respectively. When n_(a)=n_(s)=n_(x)=n_(y)=n, an (n_(a),n_(s))-asymmetric quorum system degenerates into an n-asymmetric quorumsystem. Note an asymmetric quorum system applicable to AQPS protocolsensures the intersection when clocks shift between stations.

Existing quorum schemes find an optimal coterie by either usingexhaustive searches or assuming n=k2+k+1, where k is a prime power.Observe that in clustered vehicular networks, the majority of thevehicles are members. Since these members adopt a-quorums to contacttheir clusterheads, any reduction in the size of a-quorums can yieldsignificant gain in overall energy saving. We may alternatively find anasymmetric quorum system that ensures the minimum a-quorum size.

The Majority-Based Constructing Scheme

This section presents a constructing scheme for the asymmetric quorumsystem, named Asymmetric Majority Quorum (AMQ) scheme, that has O(1)running-time complexity and gives the minimum (i.e., O(1)-sized)a-quorums. Different from most existing quorum schemes which take thecycle length n as the input, the AMQ scheme takes the delay requirementsof vehicles as the input. The produced a-quorums and s-quorums cantherefore ensure real-time neighbor discovery and transmissions betweenall stations (including clusterheads, relays, members, and RSUs). Thisscheme is tailored for environments with strict delay requirements.

Given two positive integers a and p which denote the maximally tolerableneighbor discover time (in terms of the number of beacon intervals) forthe asymmetric and symmetric links respectively. We assume α≧β sincevehicles in the same cluster are relatively stable as compared withtheir absolute moving speed. Let n=└(α−1)/2┘ be the cycle length ofmembers and U={0,1, . . . ,n−1 } be a universal set, the set of numbersof beacon intervals in a cycle, we define a generating set of a-quorumsover U as

A(α)={0}.   (1)

Let m=└(α−1)/2┘+└(β−1)/2┘−1 be the cycle length of the clusterheads (orrelays, RSUs) and U′={0,1 . . . ,m−1 } be a universal set, the set ofnumbers of beacon intervals in a cycle, we define a generating set ofs-quorums over U′, S(α,β), as

S(α,β)={0,1, . . . , [(α−1)/2]−1}.   (2)

For example, suppose α=12 and β=9. We have A(12)={0} over U={0, 1, . . ., 4} and

S(12, 9)={0, 1, . . . , 4} over U′={0, 1, . . . , 7}. Note thecardinality of A(α) equals 1 and is independent of αand β. In thepresent disclosure, we consider nontrivial parameters α, β≧5.

In the following description, we show how the AMQ scheme, i.e., Eqs. (1)and (2), can be applied to DSRC-AA to provide both the power saving andperformance guarantee to vehicular networks.

The DSRC-AA Protocol

This description introduces the DSRC Asymmetric and Asynchronous wakeup(DSRC-AA) protocol. We show that DSRC-AA is able to achievesignificantly better energy conservation than traditional AQPSprotocols. In addition, the delay incurred by DSRC-AA is bounded by thedelay requirements α and β.

Protocol Design

In one embodiment, DSRC-AA inherits the beacon structure of traditionalAQPS protocols described above. However, different from most AQPSprotocols, DSRC-AA allows different cycle patterns to be formed ondifferent stations. Specifically, we let the members of a cluster formtheir cycle patterns based on the generating set of a-quorums A(α), andlet the clusterheads, relays, and RSUs form their cycle patterns basedon the generating set of s-quorums S(α,β). During the beacon intervalswhose numbers are specified in A(α) or S(α,β), a station is required toremain fully awake. During the rest of beacon intervals, the station maysleep after the ATIM window as in the IEEE PS mode if there is no datatransmission. This process repeats so the cycle lengths on a member anda clusterhead (or relay, RSU) are n and m respectively, where

n=[(α−1)/2] and

m=[(α−1)/2]+[(β−1)/2]−1.

Each member has the duty cycle

$\begin{matrix}\frac{\overset{\_}{BI} + {\left( {n - 1} \right)\overset{\_}{AW}}}{n\overset{\_}{BI}} & (3)\end{matrix}$

and each clusterhead (or relay, RSLD has the duty cycle

$\begin{matrix}\frac{\left. {{\left\lbrack {\left( {\alpha - 1} \right)/2} \right\rbrack \overset{\_}{BI}} + \left( {m - {\left\lbrack {\alpha - 1} \right)/2}} \right\rbrack} \right)\overset{\_}{AW}}{m\overset{\_}{BI}} & (4)\end{matrix}$

Each station broadcasts its beacon frames carrying the information aboutthe awake/sleep schedule (e.g., α, β, and number of the current beaconinterval, etc.) during the ATTM windows. Once two nearby stations hearmutual beacon frames, each of them can predict one another's ATIM windowand then start the ATIM notification procedure to transmit data. Thedata buffering time in DSRC-AA may be no more than a BI (100 ms bydefault). Next, we show that DSRC-AA using A(α) and S(α,β)guarantees theworst case neighbor discovery time α BI and β BI for the asymmetric andsymmetric links respectively. We define a useful notation:

Definition 4.1 ((n, n′, l)-revolving set): Given integers n, n′, and 1,where 0≦l≦n−1. Let U={0, 1, . . . , n−1} be a universal set and Q be asubset of U. We define an (n, n′, 1)-revolving set of Q asR_(n,n′,1)(Q)={(q+in)−1:0≦(q+in)−1≦n′−1, ∀qεQ, iεZ}.

Intuitively, a revolving set is a projection of Q from the modulo-n ontothe modulo-n′ plane with an index shift l. For example, considerA(12)={0} and S(12, 9)={0, . . . , 4} shown above, which are subsets ofU={0, 1, . . . , 4} and U′={0, 1, . . . , 7} respectively. Given twoshift indices l=1 and l′=2, we may project these two sets from themodulo-5 and -8 planes onto the same modulo-11 plane by usingR_(5,11,1)(A(12))={4, 9} and R_(8,11,2)(A(12,9))={0, 1, 2, 6, 7, 8, 9,10} respectively, as we can see in FIG. 4A. Note both R_(5,11,1)(A(12))and R_(8,11,2)(A((12, 9)) are subsets of a new universal set U″={0, 1, .. . , 10}. For convenience, we denote all possible projections of Q fromthe modulo-n onto the modulo-n′ plane by R_(n,n′)={R_(n,n′,l)(Q):∀lεU}.

Lemma 4.1; Given α and β, α≧β, the pair (R_(n,α−1)(A(α)),R_(m,α−1)(S(α,β)) forms an (α−1)-bicoterie.

Proof: For brief, let A and S denote A(α) and S(α,β) respectively. Weshow that for all i and j, 0≦i≦n−1 and 0≦j≦m−1, R_(n,α−1,j)(S)≠∀. Bydefinition of A, any two elements in R_(n,α−1,i,j)(A) must have distance└(α−1)/2┘. Similarly, any two elements in R_(m,α−1,j)(S) must havedistance either 1 or └(β−1)/2┘. We can see that both R_(n,α−1,i)(A) andR_(m,α−1,j)(S) are not empty. Specifically, since α≧2└(α−1)/2┘, thereexist at least two elements in R_(n,α−1,i)(A). Let x and y be the firstand second elements of R_(n,α−1,i)(A), as shown in FIG. 4A. We have0≦x≦└(α−1)/2┘−1 and

y=x+└(α−1)/2┘≦α−2.   (5)

If x is included in R_(m,α−1,j)(S), we finish the proof. Otherwise, weshow that y must be included in R_(m,α−1,j)(S). Consider the smallestelement z (FIG. 4A) in R_(m,α−1,j)(S) which is larger than x. We have

x+1≦z≦x+└(β−1)/2┘−1   (6)

because any two elements in R_(m,α−1,j)(S) must have their distance lessthan or equal to └(β−1)/2┘. By definition of S, there exists └(α−1)/2┘continuous (and ascending) elements starting from z in R_(m,α−1,j)(S).Since β≦α, by comparing Eqs. (5) and (6) we have z<y≦min{z+└(α−1)/2┘−1,α−2}, implying that y is contained in R_(m,α−1,j)(S).

Following the previous example, we have α=12 and β=9. Suppose twostations H₀ and H₁ respectively adopt A(12) and S(12, 9) to folio theircycle patterns, as shown in FIG. 4A. The above lemma shows that, givenany reference point of time t where H₀ and H₁ are in their i^(th) andj^(th) beacon intervals, these two stations are guaranteed to overlap inat least one awake beacon interval within α−1=11 beacon intervals aftert, provided that their TBTT is aligned.

Lemma 4.2: Given α and β, α≧β, the set R_(m,β−1)(S(α,β) forms a(β−1)-coterie.

Proof: Let S denotes S(α,β). We show that for all i and j, 0≦i,j≦m−1,R_(m),β_(−1,i)(S)∩R_(m),β_(−1,i)(S′)≠∪. If both R_(m),β_(−1,i)(S) andR_(m,)β_(−1,j)(S) contain the element 0, we finish the proof. Otherwise,without loss of generality let x, x≠0, be the smallest element ofR_(m,)β_(−1,i)(S), as illustrated in FIG. 4B. By definition of S, anytwo elements in R_(m,)β_(−1,i)(S) must have distance less than└(β−1)/2┘. We have 0<x≦└(β−1)/2┘−1. If x is included inR_(m,)β_(−1,j)(S), we finish the proof. Otherwise, consider the smallestelement y in R_(m,)β_(−1,j)(S) which is larger than x (FIG. 4B). Again,by definition of S we have x+1≦y≦x+└(β−1)/2┘−1. Note that in R_(m,β)_(−1,i)(S), there exist └(α−1)/2┘ continuous elements starting form x.Since α≧β and └(α−1)/2┘≧└(β−1)/2┘, y must be included inR_(m,)β_(−1,i)(S).

Again, follow the example α=12 and β=9. Two stations H₀ and H₁ adoptingS(12, 9) to form their cycle patterns are guaranteed to discover eachother within β−1=8 beacon intervals after any reference point of time,if their TBTT is aligned. Note Lemma 4.2 can be proved by observing thatboth the cardinalities of R_(m,)β_(−1,i)(S) and R_(m,)β_(−1,j)(S) exceed(β−1)/2. That is, the elements in R_(m,)β_(−1,i)(S) andR_(m,)β_(−1,j)(S) are the majorities of the values {0, 1, . . . , β−2}.By the pigeon hole principle, we conclude that R_(m,)β_(−1,i)(S) andR_(m,)β_(−1,j)(S) must intersect.

Theorem 4.1: Given α and β, α≧β, the ordered pair (R_(n∞)(A(α)),R_(m,∞)(S(α,β))) forms an (α−1, β−1)-asymmetric quorum system.

Proof: By Definitions 3.4 and 4.1, we can see that T_(α−1),(R_(n∞)(A(α)))=(R_(n,α−1)(A(α)) andT_(β−1)(R_(n∞)(S(α,β)))=R_(m,β−1)(S(α,β)). This theorem is a directconsequence from Lemmas 4.1 and 4.2.

The above theorem implies that, in synchronous environments where thereis no clock shift between stations or the clock shifts are multiples ofa BI, DSRC-AA guarantees the worst case neighbor discovery time to beless than (α−1) BI, and (β−1) BI, for asymmetric and symmetric linksrespectively. It can be also shown that the neighbor discovery time inasynchronous environments (that is, environments where TBTT are notaligned between stations) is at most one BI, more than that insynchronous environments. As a consequence, the performance requirementsα and β can be satisfied in all conditions.

The power saving performance of DSRC-AA when applied to the real-worldscenarios in vehicular networks is now described. Follow the exampledescribed above where a tolling RSU (with radio range 15 meters) mustdiscover vehicles (with absolute moving speed up to 20 m/s) within 0.7second. We have β−7. It is shown that vehicles should ideally receivewarning messages 120 meter ahead from an accident spot to avoidcollisions. Suppose the relative moving speeds of vehicles are up to 20m/s, the transmission radius of an OBU is 200 meters, and half of theconnection period is used to send the warning message. A vehicle must beable to discover its new neighbor within

$\left\lfloor {\frac{1}{2}\frac{200 - 120}{20}} \right\rfloor = 2$

seconds. We have α=20. When the grid/torus quorum scheme is used, thegrid size has to be smaller than min{α, β}=7, implying a √{square rootover (4)}×√{square root over (4)} grid, to ensure valid neighbordiscovery time between all stations. This results in a high duty cycle

$\frac{{\left( {{2\sqrt{4}} - 1} \right) \cdot 100} + {\left( {4 - {2\sqrt{4}} + 1} \right) \cdot 25}}{4 \cdot 100} = {0.81\mspace{14mu} \left( {\overset{\_}{AW} = {25\mspace{14mu} {ms}}} \right)}$

and merely 19% power saving on an idle station. On the other hand, whenthe AMQ is applied, we have A(20)={0} over U={0, 1, . . . , 8} and S(20,7)={0, 1, . . . , 8} over U′={0, 1, . . . , 10} based on Eqs. (1) and(2) respectively. By Eq. (4), the duty cycle of clusterheads, relays,and RSUs is

${\frac{9{{\cdot 100} + \left( {11 - 9} \right) - 25}}{11 \cdot 100} = 0.86},$

yielding energy saving up to 14%. The power saving effect may be limitedon these nodes as in traditional grid/torus quorum scheme. However,members adopting A(20) may have the duty cycle

$\frac{{1 \cdot 100} + {\left( {9 - 1} \right) \cdot 25}}{9 \cdot 100} = 0.33$

by Eq. (3). This gives energy saving up to 67%, which is more than atriple of that (19%) given by the grid/torus quorum scheme. DSRC-AAtakes advantages of the asymmetric nature of delay requirements invehicular networks to improve the power saving effect on the members.

Actually, the quorum size of A(α) is optimal given the delay requirementα. If a traditional symmetric quorum system is used by a member, thecycle length must be less than or equal to α, and the resultant dutycycle can be no less than O(√{square root over (α)}/α)=O(1/√{square rootover (α)}). If AMQ is used, by Eq. (1), we can see that the memberadopting A(α) must remain awake one every n=[(α−1)/2] beacon intervals.When A(α) is projected to the modulo-α plane (as we are comparing A(α)with the traditional quorum system defined over the modulo-α plane), thestation should remain [α/[(α−1)/2]]=3 every α beacon intervals in theworst cast, and the duty cycle is no more than O(3/α)=O(1/α)—an orderless than that of traditional quorum system. The quorum size has orderO(1) over the modulo-α plane.

FIGS. 5A and 5B illustrate how α and β may affect the duty cycle on aclusterhead or member. The larger the values of α and β (i.e., thelooser the requirements), the more the energy saving. Comparing FIG. 5Aand 5B we can see that when a becomes large, a member is able to sleepmore by taxing slightly heavier duty cycle on its clusterhead. Since ina typical network members form the majority of vehicles, DSRC-AA enablessubstantial improvement in average energy efficiency.

As compared with the Grid scheme, AMQ offers at least the followingadvantages from combinatory perspective: First, Grid is symmetric,implying that all stations must wake up at least O(√n) beacon intervalsper cycle; while AMQ requires member nodes to awake O(1) beaconintervals per cycle, allowing power saving even when the cycle length nis forced to be small in vehicular networks. In addition, the cyclelengths of the Grid scheme must be a square and therefore all availablecandidates (e.g., 1, 4, or 9) may have value way below the optimal cyclelength (e.g., 13) determined based on the delay requirements α and β.This results in unnecessarily high quorum ratio on each station. AMQ, onthe other hand, generates the quorums directly based on these delayrequirements and does not rely on any assumption upon the value of n,therefore, offers improved as well as tolerable energy efficiency oneach station.

The exercise of power saving mode on an RSU may depend on its servicetype. RSUs running the transactional services, which require iteratedinteractions with vehicles, should remain active. This allows RSUs todiscover each bypassing power saving vehicle (including clusterheads andmembers) within 1 beacon interval and maximize the communication time.RSUs running the broadcast services, on the other hand, can adopts-quorums to discover the bypassing clusterheads (or other stationsadopting s-quorums) within β beacon intervals. After receiving thebroadcast message, a clusterhead can forward the message to its memberswith the buffering delay at most 1 beacon interval.

Note the hidden terminal problem may impact DSRC-AA in both neighbordiscovery time and data transmission delay when beacon and data framesrespectively are interfered by the hidden nodes. Since beacon frames areusually small (under the RTS threshold), the impact on neighbordiscovery may not be severe and can be mitigated by using the beaconbackoff mechanism to avoid the collision of beacon frames. On the otherhand, as the data transmission procedure of DSRC-AA follows the DCFmechanism, the interference between data frames can be mitigated by RTSand CTS frames, with the cost of backoff delay. If data transmissioncannot start within a single beacon interval due to the DCF backoff, thesending station can may continue the backoff during the next awakebeacon interval of the receiving station, and so forth.

Performance Guarantee in Asynchronous Vehicular Networks

Under the asynchronous conditions where the clocks shift betweenstations, the neighbor discovery time of symmetric and asymmetric linksre less than β BI and α BI respectively in DSRC-AA of the presentdisclosure in one embodiment.

Theorem 4.2: Two stations H0 and H1 adopting S(α, β) as the generatingset are able to discover each other within β beacon intervals despitetheir clock shifts.

Proof: Let m=[(α−1)/2]+[(β−1)/2]−1 and S=S(α, β). Consider the startingtime of an arbitrary beacon interval, t. Without loss of generality,assume H1's clock leads H0's clock by l+δ beacon intervals at t, where lis an integer and 0≦δ<1.

Case 1: If δ=0, let b be the number (of the current beacon interval) onH0 at t. With the clock shift l the number on H1 equals b′=(b−l) mod m.From Lemma 4.2, we know that R_(m,β−1,b)(S) and R_(m,β−1,b′)(S)intersect. Therefore H0 and H1 are able to receive one another's beaconframe within β−1 beacon intervals after t, less than β beacon intervals.

Case 2: If 0<δ<1, let p be the number of the intersected beacon intervalbetween R_(m,β−1,b)(S) and R_(m,β−1,b′)(S) and t_(s) be the startingtime of p on H0, as shown in FIG. 6. Recall in DSRCAA, H0 remains awakeduring the time interval [t_(s), t_(s)+ BI+ AW]. Since a beacon framemust be broadcasted before the end of an ATIM window, H1 may transmitits beacon frame during the time interval [t_(s)+δ BI, t_(s)+ BI+ AW].We have [t_(s)+δ BI, t_(s)+δ BI+ AW]⊂[t_(s), t_(s)+ BI+ AW]. Thisimplies that H0 is able to receive H1's beacon frame within β−1+δ beaconintervals after t, which is less than β beacon intervals.

The proof of another direction that H1 is able to receive H0's beaconframe is similar to the above arguments (consider δ′=1−δ).

Theorem 4.3: Two stations H0 and H1 adopting A(α) and S(α, β)respectively as generating sets are able to discover each other within abeacon intervals despite their clock shifts.

Proof: The proof of this theorem is based on Lemma 4.1 and is analogousto that given in Theorem 4.2.

Recall that in DSRC-AA, a station must remain awake during the ATIMwindows. The data buffering time is bounded by a BI, which satisfies thedelay requirements of data transmissions as discussed above.Additionally, with the above theorems ensuring the satisfactory neighbordiscovery time, we can see that DSRC-AA renders great feasibility forvehicular networks since it improves the energy efficiency while givingthe bounded delay.

FIG. 7A compares the duty cycles of S(α,β), Grid, and connecteddominating set (CDS), where α and β vary from 3 to 10. All the dutycycles are larger than 0.5, which gives limited effects on power saving.Note the Grid scheme, although having a smaller asymptotic quorum sizethan S(α,β) theoretically, results in the highest duty cycle because ofits sparse configuration density—it produces quorums only when the cyclelength is a square (or a compound). It can be seen that when the cyclelength is forced to be small, configuration density is a major factordetermining the energy efficiency.

FIG. 7B shows the duty cycles of A(α) where a varies from 3 to 20. Forall values of α, A(α) achieves duty cycle much smaller than CDS andGrid. The lower-bound of duty cycle in conventional quorum systems isalso illustrated in FIG. 713. Since the quorum size of A(α) alwaysequals 1, the duty cycle is not limited by the bound. From FIG. 7A andFIG. 7B, it can be seen that the improvement given by A(α) becomessignificant when the difference between 60 and β increases.

In one aspect, the methodology of the present disclosure may beimplemented as a firmware as part of the MAC layer protocols in OBU andRSU chips. Thus, DSRC-AA may improve the energy efficiency at the MAClayer and may be compatible with most existing clustering schemes at thenetwork or application layer. In addition, the power saving can beachieved at different layers with different techniques. For instance,proposed DSRC-AA protocol is orthogonal to the power saving efforts ateither PHY layer (e.g., ultra low-power wakeup radios) orRouting/Application layers (e.g., itinerary-based message propagation).

DSRC-AA of the present disclosure in one aspect provides a new powersaving protocol for vehicular networks, which improves the energyefficiency of stations (e.g., OBUs, portable devices, and RSUs) whileguaranteeing the bounded delay. Utilizing the asymmetric, clusteredvehicular network topology, DSRC-AA differentiates the awake/sleepschedules of nodes in a cluster and employs the AMQ scheme to define twotypes of cycle patterns for the members and clusterhead (or relays,RSUs) respectively. Members with the newly defined cycle patterns mayhave duty cycle below the O(1/√n) bound existing in most traditionalAQPS protocols. Since members are the majority of nodes in vehicularnetworks, DSRC-AA allows substantial reduction in average energyconsumption.

The constructions of cycle patterns are based on specific delayrequirements α and β, which denote the maximum allowable neighbordiscovery time in vehicle-to-vehicle and vehicle-to-roadsidecommunications respectively. DSRC-AA, based on the newly defined cyclepatterns, provides the asymmetric links for members to contact theirclusterheads, and the symmetric links for clusterheads (or relays, RSUs)to communicate with each other. DSRC-AA ensures that: 1) each memberusing the asymmetric link can discover its clusterhead within α beaconintervals; 2) clusterheads using the symmetric links are able todiscover each other within β beacon intervals. The data buffering timein both types of links is less than 1 beacon interval. The power savingadvantage of DSRC-AA comes with the performance guarantee. DSRC-AA, witha further generalized AMQ scheme, allows each member to dynamicallyadapt its cycle pattern according to its own delay requirement α. Thisenables more power saving on members having slow relative moving speedto their clusterheads.

Simulation results showed that DSRC-AA is able to yield more than 44%reduction in average energy consumption as compared with the existingAQPS protocols. The power saving advantage of DSRC-AA is significantunder light traffic load and high node mobility.

As will be appreciated by one skilled in the art, the present inventionmaybe embodied as a system, method or computer program product.Accordingly, the present invention may take the form of an entirelyhardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module” or “system.”

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”. “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be farther understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements, if any, in the claims below areintended to include any structure, material, or act for performing thefunction in combination with other claimed elements as specificallyclaimed. The description of the present invention has been presented forpurposes of illustration and description, but is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The embodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

Various aspects of the present disclosure may be embodied as a program,software, or computer instructions embodied in a computer or machineusable or readable medium, which causes the computer or machine toperform the steps of the method when executed on the computer,processor, and/or machine. A program storage device readable by amachine, tangibly embodying a program of instructions executable by themachine to perform various functionalities and methods described in thepresent disclosure is also provided.

The system and method of the present disclosure may be implemented andrun on a general-purpose computer or special-purpose computer system.The computer system may be any type of known or will be known systemsand may typically include a processor, memory device, a storage device,input/output devices, internal buses, and/or a communications interfacefor communicating with other computer systems in conjunction withcommunication hardware and software, etc.

The terms “computer system” and “computer network” as may be used in thepresent application may include a variety of combinations of fixedand/or portable computer hardware, software, peripherals, and storagedevices. The computer system may include a plurality of individualcomponents that are networked or otherwise linked to performcollaboratively, or may include one or more stand-alone components. Thehardware and software components of the computer system of the presentapplication may include and may be included within fixed and portabledevices such as desktop, laptop, server. A module may be a component ofa device, software, program, or system that implements some“functionality”, which can be embodied as software, hardware, firmware,electronic circuitry, or etc.

The embodiments described above are illustrative examples and it shouldnot be construed that the present invention is limited to theseparticular embodiments. Thus, various changes and modifications may beeffected by one skilled in the art without departing from the spirit orscope of the invention as defined in the appended claims.

1. A method for an asymmetric and asynchronous energy conservationprotocol for vehicular networks, comprising; defining an a-quorum forone or more members of a cluster, the cluster including a plurality ofmoving vehicles in a vehicular network and in which one or more of theplurality of moving vehicles are designated as a member, a cluster head,or a relay, the a-quorum specifying a beacon interval for one or moremembers of the cluster to establish asymmetric link to contact a clusterhead of the cluster; and defining an s-quorum for the cluster head toestablish symmetric link between the cluster head and one or more ofother cluster heads in one or more other clusters.
 2. The method ofclaim 1, wherein the cluster head adopts the s-quorum to establishsymmetric link between the cluster head and the relay.
 3. The method ofclaim 1, wherein the a-quorum is defined as having a cardinality of one.4. The method of claim 1, wherein the a-quorum is defined as being 0-thbeacon interval.
 5. The method of claim 1, wherein the one or moremembers adopting the a-quorum is bounded by a delay requirement α. 6.The method of claim 1, wherein the s-quorum is defined asS(α,β)={0,1, . . . , [(α−l)/2]−1}, wherein α denotes neighbor discovertime in beacon intervals for vehicle-to-vehicle asymmetric links, βdenotes neighbor discovery time in beacon intervals forvehicle-to-vehicle symmetric links.
 7. The method of claim 1, whereinthe cardinality of a-quorum equals one and is independent of theneighbor discover times for vehicle-to-vehicle asymmetric and symmetriclinks.
 8. The method of claim 1, wherein different cycle patterns areformed on different vehicles of the plurality of vehicles.
 9. The methodof claim 1, wherein the members of the cluster form cycle patterns basedon a set of the a-quorum.
 10. The method of claim 1, wherein the clusterhead, the relay, and one or more RSUs form cycle patterns based on thes-quorum.
 11. The method of claim 1, wherein the a-quorum is defined asA(α)={0} and the s-quorum is defined as S(α,β)={0,1, . . . ,[(α−1)/2]−1}, wherein α denotes neighbor discover time in beaconintervals for vehicle-to-vehicle asymmetric links, δ denotes neighbordiscovery time in beacon intervals for vehicle-to-vehicle symmetriclinks, and wherein the member remains awake during a beacon intervalspecified in A(α) and the cluster head and the relay remain awake duringa beacon interval specified in S(α,β).
 12. The method of claim 11,wherein the one or more members broadcast their beacon frames carryinginformation about awake and sleep schedule during ATIM windows.
 13. Themethod of claim 12, wherein one or more members transmit data based onthe information.
 14. A program storage device readable by a machine,tangibly embodying a program of instructions executable by the machineto perform a method of an asymmetric and asynchronous energyconservation protocol for vehicular networks, comprising: defining ana-quorum for one or more members of a cluster, the cluster including aplurality of moving vehicles in a vehicular network and in which theplurality of moving vehicles are designated as a member, a cluster head,or a relay, the a-quorum specifying a beacon interval for one or moremembers of the cluster to establish asymmetric links to contact acluster head of the cluster; and defining an s-quorum for the clusterhead to establish symmetric link between the cluster head and one ormore of other cluster heads in one or more other clusters.
 15. Theprogram storage device of claim 14, wherein the cluster head adopt thes-quorum to establish symmetric link between the cluster head and therelay.
 16. The program storage device of claim 14, wherein the a-quorumis defined as having a cardinality of one.
 17. The program storagedevice of claim 14, wherein the a-quorum is defined as being 0-th beaconinterval.
 18. The program storage device of claim 14, wherein the one ormore members adopting the a-quorum is bounded by a delay requirement α.19. The program storage device of claim 14, wherein the s-quorum isdefined asS(α,β)={0,1, . . . , [(α−1)/2]−1}, wherein α denotes neighbor discovertime in beacon intervals for vehicle-to-vehicle asymmetric links, βdenotes neighbor discovery time in beacon intervals forvehicle-to-vehicle symmetric links.
 20. A system for an asymmetric andasynchronous energy conservation protocol for vehicular networks,comprising: means for defining an a-quorum for one or more members of acluster, the cluster including a plurality of moving vehicles in avehicular network and in which the plurality of moving vehicles aredesignated as a member, a cluster head, or a relay, the a-quorumspecifying a beacon interval for one or more members of the cluster toestablish asymmetric links to contact a cluster head of the cluster; andmeans for defining an s-quorum for the cluster head to establishsymmetric link between the cluster head and one or more of other clusterheads in one or more other clusters.
 21. The system of claim 20, whereinthe cluster head adopt the s-quorum to establish symmetric link betweenthe cluster head and the relay.
 22. The system of claim 20, wherein thea-quorum is defined as having a cardinality of one.
 23. The system ofclaim 20, wherein the a-quorum is defined as being 0-th beacon interval.24. The system of claim 20, wherein the one or more members adopting thea-quorum is bounded by a delay requirement α.
 25. The system of claim20, wherein the s-quorum is defined asS(α,β)={0,1, . . . , [(α−1)/2]−1}, wherein α denotes neighbor discovertime in beacon intervals for vehicle-to-vehicle asymmetric links, βdenotes neighbor discovery time in beacon intervals forvehicle-to-vehicle symmetric links.
 26. A system for an asymmetric andasynchronous energy conservation protocol for vehicular networks,comprising: a plurality of stations associated with respective one ormore vehicles in a vehicular network, the plurality of stations beingorganized into one or more clusters, and assigned as one of a member,cluster head, or relay in a respective cluster; an a-quorum specifying abeacon interval for one or more members of a cluster to establishasymmetric links to contact a cluster head of the cluster; and ans-quorum for the cluster head to establish symmetric link between thecluster head and one or more of other cluster heads in one or more otherclusters.
 27. The system of claim 26, wherein the cluster head adopt thes-quorum to establish symmetric link between the cluster head and therelay.
 28. The system of claim 26, wherein the a-quorum is defined ashaving a cardinality of one.
 29. The system of claim 26, wherein thea-quorum is defined as being 0-th beacon interval.
 30. The system ofclaim 26, wherein the s-quorum is defined asS(α,β)={0,1, . . . , [(α−1)/2]−1}, wherein α denotes neighbor discovertime in beacon intervals for vehicle-to-vehicle asymmetric links, βdenotes neighbor discovery time in beacon intervals forvehicle-to-vehicle symmetric links.